A method of forming a thermal protective coating on a super alloy metal substrate

ABSTRACT

A method of forming a gas turbine part includes forming a bonding underlayer on a superalloy metal substrate, the underlayer including an intermetallic compound of aluminum, nickel, and platinum, and forming a ceramic outer layer on the alumina film formed on the bonding underlayer. The bonding underlayer essentially comprises an Ni—Pt—Al ternary system constituted by an aluminum-enriched α-NiPt type structure, in particular an Ni—Pt—Al ternary system having a composition Ni z Pt y Al x  in which z, y, and x are such that 0.05≦z≦0.40, 0.30≦y≦0.60, and 0.15≦x≦0.40.

CROSS REFERENCE OF RELATED APPLICATION

This application is a Divisional of patent application Ser. No.10/974,973 filed Oct. 28, 2004, which is incorporated herein byreference.

BACKGROUND OF INVENTION

The invention relates to making a protective coating on a superalloymetal substrate.

The field of application of the invention is making parts that arecapable of retaining mechanical strength at high temperatures, inparticular gas turbine parts, such as turbine blades, in particular forturbojets.

In order to improve performance, and in particular efficiency, it isdesirable to cause gas turbines to operate at temperatures that are ashigh as possible.

In order to make parts for the hot portions, it is common practice touse superalloys. These alloys usually comprise nickel as the mainconstituent and additional elements generally selected from chromium,cobalt, aluminum, molybdenum, titanium, tantalum, and many others.

An increase in operating temperature is made possible by providing themetal substrate of such parts with a protective coating forming athermal barrier.

It is known to make a protective coating comprising an ceramic outerlayer and a bonding underlayer of metal, in particular a bondingunderlayer containing aluminum and at least one other metal, such asplatinum.

The bonding underlayer interposed between the superalloy metal substrateand the ceramic outer layer performs the following functions:

it enables a film of alumina to form and to persist at its surface,thereby enhancing bonding with the ceramic outer layer;

it protects the substrate from corrosion by oxidation by the oxygen inthe ambient medium that manages to pass through the outer ceramic layer;and

it constitutes a diffusion barrier against certain elements of the metalsubstrate which would otherwise contaminate the film of alumina, andwould consequently affect the interface between the bonding underlayerand the outer ceramic layer, thereby affecting the adhesion thereof.

Including reactive elements such as yttrium, cerium, hafnium, orlanthanides in the bonding underlayer reinforces its diffusion barrierfunction and enhances the persistence of the “adhesive” film of alumina.

It is well known to form a bonding underlayer of the MCrAlY type (whereM is a metal such as Fe, Ni, Co) by a method such as plasma projection,without inducing a reaction with the substrate, the adhesion of thebonding underlayer on the substrate being of a mechanical kind.Reference can be made, for example, to U.S. Pat. Nos. 4,055,704 and5,824,423. Nevertheless, in order to obtain an under-layer that isthermally stable, it is necessary to give it relatively large thickness,typically at least a thickness lying in the range 50 micrometers (μm) to100 μm, and such thickness is penalizing in terms of weight.

Other known methods consist in making a bonding underlayer out of anintermetallic compound, which can be of smaller thickness because of itsthermal stability. An intermetallic compound comprising aluminum andplatinum has been found to have good properties.

Thus, U.S. Pat. Nos. 5,716,720 and 5,856,027 describe a methodconsisting in electroplating a layer of platinum on a substrate made ofa nickel-based superalloy, and subsequently in performing vaporaluminization at a temperature greater than 1000° C. Nickel coming fromthe substrate diffuses into the bonding underlayer. An alumina film isformed by heat treatment at the surface of the bonding underlayer priorto forming a ceramic outer layer, e.g. of yttria stabilised zirconiaobtained by physical vapor deposition (PVD).

A reactive element may be introduced into the bonding underlayer duringthe vapor aluminization step. In its outer portion surmounting adiffusion zone in the vicinity of the substrate, the bonding underlayerpresents an intermediate phase comprising 18% to 28% by weight aluminum,50% to 60% by weight nickel, and 8% to 35% by weight platinum,corresponding to a β-type solid solution phase in the binarynickel-aluminum phase diagram (β-NiAl).

U.S. Pat. No. 5,238,752 describes another method consisting in formingon a superalloy substrate a bonding underlayer made of an intermetalliccompound, in particular a compound of aluminum and platinum. The bondingunder-layer is made by pack cementation at a temperature higher than985° C. and at a thickness greater than 25 μm. An alumina film is formedby oxidation at the surface of the bonding underlayer prior to forming aceramic outer layer, e.g. of yttria stabilised zirconia by physicalvapor deposition.

European patent application EP 0 985 744 describes yet another methodcomprising forming a layer of platinum on a nickel-based superalloysubstrate by electroplating or by chemical vapor deposition, anddepositing a layer of aluminum which is made from a gaseous halide andwhich diffuses into the layer of platinum. Sulfur is removed after eachdeposit by heat treatment at a temperature higher than 1050° C. withsurface descaling so as to eliminate sulfur which is harmful to adhesionof the alumina film developed on the surface of the resulting bondingunderlayer.

US patent application No. US 2002/0037220 discloses a method in whichthe bonding underlayer is formed by physical vapor deposition of aplurality of individual layers alternately of aluminum and of a metalfrom the platinum group, and by exothermal reaction between the metalsof the layers formed. By using a physical vapor deposition method, thetemperature of the substrate is relatively low, and remains at a valuewell below that from which the elements of the substrate are liable todiffuse into the deposit being formed.

OBJECT AND SUMMARY OF THE INVENTION

An object of the invention is to provide a gas turbine part comprising asuperalloy metal substrate provided with a protective coating having abonding under-layer between the substrate and the ceramic outer layerthat is stable and that presents long-term resistance to spalling of theceramic layer, while being small in thickness and therefore light inweight.

This object is achieved by a gas turbine part comprising a superalloymetal substrate, a bonding underlayer formed on the substrate andcomprising an intermetallic compound comprising aluminum, nickel, andplatinum, and a ceramic outer coating anchored on a film of aluminaformed on the bonding underlayer,

in which, in accordance with the invention, the bonding underlayeressentially comprises an Ni—Pt—Al ternary system constituted by analuminum-enriched α-NiPt type structure.

A characteristic of the invention lies in at least a majority of thebonding underlayer being constituted by a solid solution phase of a typein the nickel (Ni) and platinum (Pt) binary phase diagram and alsoincorporating aluminum (Al).

It is because of the great stability of such a phase that a bondingunder-layer can be made, even with small thickness, without affectingthe properties, and in particular the robustness, of the thermalprotection. The thermal protection presents increased resistance tospalling, even after repeated thermal cycles.

In addition, with a substrate made of a nickel-based superalloy, thediffusion of nickel over time from the substrate into the bonding layercan modify the composition of the bonding layer, but not its structure,and thus cannot modify the stability of the α-NiPt intermetalliccompound, with this being more marked the closer the initial nickelcontent is to its minimum value in the α-NiPt domain.

An α-type NiPt solid solution phase that is enriched in Al is itselfknown and can be characterized by its crystallographic structure asdescribed in particular in an article by Janice L. Kann et al. entitled“Phase stability in (Ni,Pt)₃Al alloys” and published in ScriptaMetallurgica et Materiala, Vol. 31, No. 11, pp. 1461-1464, 1994.

A reference to such a phase can also be found in an article by B.Gleeson et al. entitled “Effects on platinum on the interdiffusion andoxidation behavior of Ni—Al-based alloys”, published in the Proceedingsof the 6th International Symposium on High Temperature Corrosion andProtection of Materials, Materials Science Forum, Vols. 461-464, pp.213-222, 2004.

Advantageously, the Ni—Pt—Al ternary system has a compositionNi_(z)Pt_(y)Al_(x) in which z, y, and x are such that 0.05≦z≦0.40,0.30≦y≦0.60, and 0.15≦x≦0.40.

The bonding underlayer may comprise one or more additional metals otherthan Al, Ni, and Pt, and in particular at least one metal elected fromchromium and cobalt which contributes to stability, and/or at least oneprecious metal selected from palladium, ruthenium, and rhenium.

The bonding underlayer may also include at least one reactive elementselected from the group constituted by yttrium, zirconium, hafnium, andthe lanthanides.

In all cases, the majority of the bonding underlayer remains formed bythe ternary Ni—Pt—Al system, which preferably represents at least 75%(atomic percentage) of the composition of the underlayer.

The thickness of the bonding underlayer advantageously lies in the range2 μm to 120 μm, and is preferably less than 40 μm. It is possible tochoose a small thickness, less than 40 μm, because of the stabilityconferred by the presence of the α-NiPt phase, thereby limiting thefabrication cost and the mass of the bonding underlayer.

Another object of the invention is to provide a method of forming athermal protective coating constituting a thermal barrier on asuperalloy metal substrate with a bonding underlayer which is stable andcan have a reduced mass.

This object is achieved by a method comprising forming a bondingunderlayer on the substrate, said underlayer comprising an intermetalliccompound of aluminum, nickel, and platinum, and forming a ceramic outerlayer which is anchored on an alumina film present on the bondingunderlayer,

in which method, in accordance with the invention, a bonding under-layeris formed that essentially comprises an Ni—Pt—Al ternary systemconstituted by a structure of the aluminum-enriched α-NiPt type.

In an implementation of the method, the bonding underlayer is made byforming on the substrate a coating of composition corresponding to thecomposition desired for the underlayer. In which case, a coatingformation process is used that is of the overlay type, i.e. that doesnot involve in substantial or significant manner any diffusion ofelements coming from the substrate into the coating.

Amongst the processes that are suitable, mention can be made of physicalvapor deposition, deposition by cathode sputtering, or by plasmaprojection, and electro-deposition.

The bonding underlayer may thus be formed by physical vapor depositionof at least a plurality of individual layers respectively of platinum,nickel, and aluminum, and by causing the metals of the deposited layersto react together.

It is also possible to deposit at least some layers including aplurality of components of the bonding underlayer in pre-alloyed form,for example it is possible to deposit pre-alloyed layers such as NiPt orPtAl in alternation with layers of Al or Ni.

It is also possible to envisage forming the bonding underlayer byphysical vapor deposition from a source having a compositioncorresponding to that which is desired for the bonding underlayer, forexample by plasma projection from a pre-alloyed powder mixture.

In another implementation of the invention, the bonding underlayer isformed on a nickel-based superalloy substrate by physical vapordeposition of at least a plurality of alternating individual layers ofplatinum and of aluminum, by causing the metals of the layers depositedat a moderate temperature, i.e. lower than 900° C., and typically about700° C., to react exothermally, and by heat treatment so as to causenickel from the substrate to diffuse into the bonding underlayer. Theheat treatment can be performed separately from forming the ceramicouter layer or it can be the result of forming the ceramic outer layerwhen it is made at relatively high temperature. It has been found by theApplicant that heat treatment at a temperature of not less than 900° C.suffices to cause nickel to diffuse from the substrate throughout thebonding underlayer providing the underlayer is of relatively smallthickness, e.g. less than or equal to 10 μm, with a higher temperaturepossibly being required if the thickness is greater. It has also beenfound that such diffusion leads to a stable phase of the α-NiPt typebeing formed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the followingdescription given by way of non-limiting indication.

Reference is made to the accompanying drawings, in which:

FIG. 1 is a highly diagrammatic section view of a superalloy metalsubstrate provided with a protective coating;

FIG. 2 is a diagram of a lattice cell characteristic of analuminum-enriched α-NiPt solid solution phase;

FIG. 3 shows an implementation of a method of the invention;

FIGS. 4 to 6 show examples of implementations of a first step in themethod of FIG. 3;

FIG. 7 shows another implementation of the method of the invention;

FIG. 8 is a photograph taken with scanning electron microscope andshowing a cross-section of a protective coating made in accordance withan implementation of the invention on a superalloy metal substrate;

FIG. 9 shows the results of tests performed on parts made with coatingsof the invention and in accordance with the prior art; and

FIGS. 10 and 11 are photographs taken with a scanning electronmicroscope showing a cross-section of the protective coating made usinganother implementation of the invention, both before and after moderateheat treatment.

DETAILED DESCRIPTION OF THE INVENTION

The description below relates to making a protective coating on asuperalloy metal part, typically a gas turbine part, such as a turbineblade for a turbojet.

The protective coating is of the type shown highly diagrammatically inFIG. 1. On a metal substrate 10 made of superalloy, there is formed acoating comprising a bonding underlayer 12 made of an intermetalliccompound formed mainly of aluminum, nickel, and platinum, and a ceramicouter layer 16 anchored on an alumina film 14 developed on the surfaceof the bonding underlayer.

In accordance with the invention, the majority of the intermetalliccompound forming the bonding underlayer comprises an aluminum-enrichedNi—Pt—Al ternary system constituted by a phase of the α-NiPt type. Sucha phase may be defined by its lattice structure, as shown in FIG. 2.This structure is a face-centered tetragonal system of type L1 _(o).Atoms of Ni and Al are disposed at the vertices and at the centers ofthe (001) faces, while atoms of Pt are situated at the centers of the(100) and (010) faces. For one lattice cell, the dimensions a, b, and c(FIG. 2) are such that 0.37 nm≦a=b≦0.40 nm, and 0.35 nm≦c≦0.36 nm. It ispreferable to select an Ni_(z)Pt_(y)Al_(x) ternary system in which z, y,and x are such that 0.05≦z≦0.40, 0.30≦y≦0.60, and 0.15≦x≦0.40.

The domain containing the α-NiPt phase is totally separate from thedomain containing the β-NiAl phase, as is confirmed by the above-citedarticle by B. Gleeson et al.

Elements other than Ni, Al, and Pt can be added to the composition ofthe bonding underlayer, in particular reactive elements selected fromyttrium, zirconium, hafnium, and the lanthanides which serve toreinforce the diffusion barrier function against certain elements of thesubstrate that could be harmful to the strength of the protectivecoating, and which enhance the persistence of the alumina film. Othermetals having a beneficial effect may also be added, e.g. palladium,ruthenium, or rhenium, or even cobalt and/or chromium which improve thethermal stability of the coating.

The alumina film 14 is produced by oxidation of the aluminum of thediffusion barrier. It provides a protective function against corrosionby oxidation. It also provides for attachment of the ceramic outer layer16 by virtue of its “adhesive” nature.

The ceramic outer coating 16 essentially provides a thermal insulationfunction. It is made of refractory oxide, such as zirconia, yttriumoxide, or yttria stabilised zirconia. It may be formed by physical vapordeposition, e.g. by electron beam evaporation or by plasma-assistedevaporation, as is well known in the art.

The composition of the bonding underlayer may be as follows (in atomicpercentages):

in the range 75% to 100% of the above-defined Ni—Pt—Al ternary system;

in the range 0% to 10% cobalt and/or chromium;

in the range 0% to 5% reactive element(s) selected from Y, Zr, Hf, andthe lanthanides; and

in the range 0% to 10% precious metals selected from Pd, Ru, and Rh.

The thickness of the bonding underlayer preferably lies in the range 2μm to 120 μm. Because of the great thermal stability conferred by theα-NiPt phase, this thickness may advantageously be less than 40 μm, oreven less than 20 μm.

First Implementation

In a first implementation (FIG. 3), the bonding underlayer is formed bymaking a coating having the composition desired for the bondingunderlayer, without causing substantial or significant diffusion ofelements coming from the substrate, i.e. by implementing an overlay typeprocess (step 20).

Thereafter, (step 22), an outer layer is formed on the bondingunder-layer with growth of the alumina film that develops at the surfacethereof. For this purpose, it is possible to use a physical vapordeposition method under an electron beam (EB-PVD). The metallicsubstrates carrying the bonding underlayer are placed in an enclosureabove a source of ceramic, e.g. a yttria stabilised zirconia target.Deposition is performed under a reduced atmosphere comprising a mixtureof argon and oxygen by exciting an electron gun facing the source ofceramic. That method is well known in the art.

Step 20 can be performed in several ways, in particular by implementinga physical vapor deposition process such as cathode sputtering, electronbeam PVD, or evaporation under an arc, with or without assistance from aplasma.

In a first variant (FIG. 4), step 20 comprises M repeats of a sequenceof depositions of single layers of platinum (step 201), aluminum (step202), platinum (step 203), and nickel (step 204), followed by a finaldeposition of a single layer of platinum (step 205). Moderate heattreatment (step 206) can then be performed prior to forming the outerlayer of ceramic, so as to form an intermetallic compound by causingreaction to take place between the metals of the single layers. Heattreatment is performed at a temperature lower than 900° C., e.g. about700° C., so that no diffusion of elements from the substrate into theadjacent portion of the intermetallic compound is encouraged. The heattreatment is performed in a vacuum or in an inert atmosphere for aduration lying in the range 0.5 hours (h) to 3 h, for example.

It should be observed that the moderate heat treatment is optional, andthe intermetallic compound can also be formed under the effect of thetemperature rise that occurs while making the ceramic outer layer.

It should also be observed that in the sequence of depositing singlelayers, a single layer of aluminum is preferably placed between twosingle layers of platinum in order to avoid any reaction betweenaluminum and nickel which could disturb the diffusion of platinum in theintermetallic compound. The first single layer is a layer constitutedessentially by platinum since that is less likely to diffuse into thesubstrate. The last single layer is also preferably a layer of platinumsince that is less likely to oxidize in air or in a partial pressure ofoxygen at the end of making the bonding underlayer.

The single layers are made to have individual thicknesses, at least foraluminum, of less than 2000 nanometers (nm), and preferably of no morethan 1500 nm. Layer thickness may be selected to be well below thisthreshold, for example no more than 200 nm. Such a relatively smallthickness is selected if, after heat treatment, it is desired to obtaina structure that is homogenous, i.e. that does not leave any trace ofthe bonding underlayer being built up from superposed layers.

The number M of sequences is determined as a function of the thicknessesof the single layers and of the total thickness desired for the bondingunderlayer. Depending on the value of said total thickness, the numberof single layers may vary over the range a plurality of units to aplurality of tens, or even a plurality of hundreds.

It should be observed that the single layers, as deposited, may be ofdifferent thicknesses.

In all cases, the ratios between the total thickness of the layers foreach metal are a function of the composition desired for theintermetallic compound forming the bonding underlayer.

In a second variant (FIG. 5), step 20 comprises N repeats of a sequenceof single layer depositions of a binary system, e.g. an NiPt (step 210),and of aluminum (step 211), prior to optional moderate heat treatment(step 206) as in the process of FIG. 4. Naturally, in a variant, it ispossible to alternate depositing a PtAl binary system and nickel Ni. Thecomposition of the NiPt binary system, the thicknesses of the singlelayers and the numbers thereof, are selected as a function of thecomposition and the thickness desired for the bonding underlayer.

In a third variant (FIG. 6), step 20 comprises successively depositing Players of an Ni—Pt—Al ternary system (step 215) prior to optionalmoderate heat treatment (step 206).

Each layer is given a composition corresponding to the compositiondesired for the bonding underlayer.

In the processes of FIGS. 4 to 6 as described above, in particular whenusing a PVD process, sources or targets are used that are made ofnickel, of platinum, or of aluminum, or that are made of an alloy of twoof those materials, or that are made of an alloy of all three metals,the metals being present, for example, in the form of powders. Whenadditional metals or other elements are also to be incorporated in thebonding underlayer, they may be provided by additional sources ortargets so as to be deposited in distinct single layers, or they maypreviously be alloyed in the desired proportions in one or more of thenickel and/or platinum and/or aluminum sources or targets.

In yet another variant, the bonding underlayer may be formed byelectro-deposition without significant interaction with the substrate.It is possible to proceed by depositing successive layers of differentmetals or by co-deposition of these metals.

Second Implementation

In a second implementation (FIG. 7), with a metal substrate made of anickel-based superalloy, the bonding underlayer is made by forming, in afirst portion of the process, an intermetallic compound essentiallycomprising aluminum and platinum, and in a subsequent portion of theprocess, by causing nickel from the substrate to diffuse by raising thetemperature before or during formation of the ceramic outer layer.

The first portion of the process can be performed by alternatingdeposits of single layers of platinum (step 30) and of aluminum (step32) using a physical vapor deposition process, and by causing anexothermal reaction to take place between the layers formed thereby. Tothis end, it is possible to use the method described in above-cited USpatent application No. US 2002/0037220.

For the reasons already given, the first single layer to be deposited onthe substrate and the last single layer to be deposited (step 34) arepreferably platinum layers.

A moderate heat treatment step 36 is performed so as to form anintermetallic compound by exothermal reaction between the platinum andthe aluminum of the single layers that have been formed. The heattreatment is performed at a temperature lower than 900° C., e.g. about700° C., so that no diffusion of elements from the metal substrate intothe adjacent portion of the intermetallic compound is encouraged. Theheat treatment is performed under a non-oxidizing atmosphere, e.g. undera vacuum or an inert atmosphere for a duration lying in the range 0.5 hto 3 h, e.g. about 2 h. During heat treatment, the aluminum in any onelayer diffuses into the adjacent layers of platinum. A thin film ofalumina develops on the surface of the bonding underlayer as obtained inthis way during subsequent exposure to an oxidizing medium.

The single layers are made to have individual thicknesses, at least foraluminum of less than 2000 nm, and preferably no greater than 1500 nm.It is possible for this thickness to be selected well below thisthreshold, e.g. no greater than 200 nm.

The thicknesses and the number of layers are selected so as to obtain anAl/Pt ratio corresponding to that which is desired in the bondingunderlayer and so as to obtain the thickness that is desired therefor.

The single layers of platinum and of aluminum may be deposited bycathode sputtering, by electron beam physical vapor deposition, or byevaporation under an arc, with or without a plasma, which processes makeit possible to control quite accurately the quantity of metal that isdeposited, and thus the thicknesses of the single layers. Such processesare well known in themselves and they use sources or targets made ofplatinum and of aluminum.

At least one additional metal and/or at least one reactive element canbe deposited within the bonding underlayer by using one or moreadditional sources or targets, or by incorporating those materials inthe sources or targets of platinum and aluminum.

Thereafter, the ceramic outer layer is made (step 37), but after raisingthe temperature of the substrate to a value that is high enough to causethe nickel contained in the metal substrate to diffuse within thebonding underlayer. This temperature should be selected to beincreasingly high for increasing thickness of the bonding underlayer. Itis preferably equal to not less than 900° C. for a thickness lying inthe range 2 μm to 10 μm, and it may exceed 1000° C. for a greaterthickness.

Other metals from the substrate are also likely to diffuse, such ascobalt and chromium. Nevertheless, the bonding underlayer conserves itsdiffusion barrier function against elements that might be contained inthe substrate, such as tungsten, molybdenum, tantalum, which could havea harmful effect on the strength of the protective coating, inparticular on the persistence of the alumina film at the surface of thebonding underlayer.

The Applicant has been able to show that nickel diffusing in the bondingunderlayer co-operates with the platinum to form a stable phase of theα-NiPt type.

In a variant, the heat treatment at a temperature of at least 900° C.,seeking to cause nickel from the substrate to diffuse into the bondingunderlayer, can be performed separately, prior to forming the ceramicouter layer.

EXAMPLE 1

Metal parts were used made of a single crystal superalloy based onnickel and having the following composition (percentages by weight):6.5% Co, 7.5% Cr, 5.3% Al, 1.2% Ti, 8% Ta, 2% Mo, 5.5% W, with thebalance being Ni.

Parts were provided with alternating layers of platinum and aluminum bya physical vapor deposition process using cathode sputtering, inapplication of the second implementation described above (FIG. 7). 84single layers of platinum were deposited each having a thickness of 30nm, alternating with 83 single layers of aluminum each having athickness of 66 nm.

Temperature was raised to 700° C. for 2 h in order to trigger anexothermal reaction between the single layers, causing a PtAl₂ typeplatinum and aluminum intermetallic compound to be formed in a layerhaving a thickness of 7.5 μm.

Thereafter, a ceramic outer layer of zirconia ZrO₂ stabilized withyttrium oxide Y₂O₃ (representing 8% by weight) was deposited. Depositiontook place as described above by electron beam physical vapordeposition. The temperature of the substrate was raised to about 1000°C. and the duration was selected so as to form an outer layer of yttriastabilised zirconia having a thickness equal to about 125 μm.

EXAMPLE 2

The procedure was the same as in Example 1, but the number of singlelayers of platinum and aluminum was limited so as to obtain, afterexothermal reaction between them, a PtAl₂ type intermetallic compound ina layer having a thickness equal to about 2.5 μm.

The microphotograph of FIG. 8 shows the results that were obtained.

EXAMPLE 3 For Comparison

On substrates having the same composition as that of Examples 1 and 2, abonding underlayer was formed by electro-deposition of a platinum layerand by vapor aluminization so as to obtain, in a manner that is known inthe prior art, a bonding underlayer corresponding to a platinum-enrichedP-type phase of the Ni—Al binary phase diagram. The thickness of thebonding underlayer was 60 μm. Thereafter, a ceramic outer layer wasformed as described in Example 1.

Tests for ability to withstand thermal cycling in an oxidizing medium(air) were performed on the parts A, B, and C as obtained in Examples 1,2, and 3 respectively, each cycle comprising a rapid temperature rise upto 1100° C. which was maintained for 1 h, after which there was a returnto ambient temperature which was maintained for 15 minutes (min).

As shown in FIG. 9, the parts B withstood 624 cycles in satisfactorymanner, which is a very remarkable result given the very small thickness(2.5 μm) of the bonding underlayer, when compared with the thicknessesof the bonding under-layers presently in use (commonest 60 μm). Parts Aand C withstood cycling in satisfactory manner up to 1086 cycles.

The possibility of using a thin bonding underlayer leads to fasterimplementation, to a saving in materials cost (even though the platinumcontent is relatively high), and to a saving in terms of weight, all ofwhich constitute significant advantages.

EXAMPLE 4

A single crystal superalloy metal part as defined in Example 1 wasprovided with layers by repeating the sequence Pt, Al, Pt, Ni with afinal layer of Pt (variant of the above-described first implementationas shown in FIG. 4). A physical vapor deposition process by cathodesputtering was used. Thirteen single layers of Pt were deposited eachhaving a thickness of 181 nm, six individual layers of Ni were depositedeach having a thickness of about 268 nm, and six individual layers of Alwere deposited each having a thickness of about 171 nm.

Moderate heat treatment at a temperature of about 700° C. under a vacuumwas performed in order to trigger a reaction between the single layerswithout leading to migration from the substrate of single crystalsuperalloy. A coating was thus obtained made of an intermetalliccompound having a thickness approximately equal to 7.1 μm. Thecomposition in atomic percentages of the coating was 45% Pt, 28% Al, and27% Ni. The coating was examined by X-ray diffraction which demonstratedthe existence of a crystallographic structure characteristic of analuminum-enriched α-NiPt phase.

FIGS. 10 and 11 show the coating, in section, respectively before andafter moderate heat treatment.

1. A method of forming a thermal protective coating on a superalloymetal substrate, the method comprising: forming a bonding underlayer onthe substrate, said underlayer comprising an intermetallic compound ofaluminum, nickel, and platinum; and forming a ceramic outer layer whichis anchored on an alumina film present on the bonding underlayer,wherein a bonding underlayer is formed comprising in majority anNi—Pt—Al ternary system constituted by aluminum-enriched α-NiPtstructure.
 2. A method according to claim 1, wherein the bondingunderlayer is made by forming on the substrate a coating of compositioncorresponding to the composition desired for the underlayer.
 3. A methodaccording to claim 2, wherein the bonding underlayer is made by forminga coating by physical vapor deposition.
 4. A method according to claim3, wherein the bonding underlayer is formed by physical vapor depositionof a plurality of single layers respectively of platinum, nickel, andaluminum, and by causing the metals of the deposited layers to reacttogether.
 5. A method according to claim 3, wherein the bondingunderlayer is formed by depositing layers, at least some of whichcomprise a plurality of components of the underlayer in pre-alloyedform.
 6. A method according to claim 3, wherein the bonding underlayeris formed by depositing a pre-alloyed composition corresponding to thecomposition desired for the underlayer.
 7. A method according to claim2, wherein the bonding underlayer is made by forming a coating byelectro-deposition.
 8. A method according to claim 1, wherein thebonding underlayer is formed on a superalloy substrate based on nickel,by physical vapor deposition of at least a plurality of alternatingsingle layers of platinum and of aluminum, by exothermal reactionbetween the metals of the deposited layers, and by heat treatment at atemperature of not less than 900° C. in order to cause nickel from thesubstrate to diffuse into the bonding underlayer.
 9. A method accordingto claim 8, wherein the heat treatment at a temperature of not less than900° C. is performed while forming the ceramic outer layer.
 10. A methodaccording to claim 8, wherein the heat treatment at a temperature of notless than 900° C. is performed prior to forming the ceramic outer layer.11. A method according to claim 1, wherein the Ni—Pt—Al ternary systemhas a composition Ni_(z)Pt_(y)Al_(x) in which z, y, and x are such that0.05≦z≦0.40, 0.30≦y≦0.60, and 0.15≦x≦0.40.